feat: implement Tier 1 IRC numerics

- AWAY command with RPL_AWAY (301), RPL_UNAWAY (305), RPL_NOWAWAY (306)
- RPL_WHOISIDLE (317) with idle time and signon time in WHOIS
- RPL_TOPICWHOTIME (333) with topic setter and timestamp
- Schema migration 002 for away_message and topic metadata columns
- Refactor deliverJoinNumerics into smaller helper functions

internal/db/schema/002_away_and_topic_meta.sql

@@ -0,0 +1,6 @@
+-- Add away message to sessions
+ALTER TABLE sessions ADD COLUMN away_message TEXT NOT NULL DEFAULT '';
+
+-- Add topic metadata to channels
+ALTER TABLE channels ADD COLUMN topic_set_by TEXT NOT NULL DEFAULT '';
+ALTER TABLE channels ADD COLUMN topic_set_at DATETIME;